GMR devices are a type of dielectric diffraction grating comprising periodically varying regions of high and low refractive index (RI) materials. At specific wavelengths, these gratings exhibit a strong resonance effect, changing the response to incident light from mostly-transmissive to mostly-reflective. Due to this resonance's sensitivity to the refractive index of surrounding materials, GMR devices have garnered a great deal of attention in recent years for their uses in a wide variety of fields.